The present invention relates in general to audio systems which prevent distortion from amplifier clipping by employing dynamic gain limiting, and more specifically to automotive audio systems for providing a consistent perception of distortion with varying spectral content of a particular audio signal.
Typical audio reproduction systems include a variable-gain amplification stage followed by a fixed-gain power amplifier which feeds an output transducer such as a speaker. A volume or gain command (e.g., a control voltage) provided to the variable gain stage controls the output volume heard by a listener.
An important objective in designing an audio system is to provide minimum distortion in signal reproduction. However, there is always some distortion, especially at high sound levels. As the magnitude of the signal provided from the variable gain amplifier stage to the power amplifier increases above a certain level, the power amplifier becomes overdriven. This situation occurs when the input signal to the power amplifier multiplied by the fixed gain of the power amplifier approaches the supply voltage level provided to the power amplifier. As a result, the power amplifier becomes saturated and signal peaks of the audio signal are distorted by clipping.
The problem of power amplifier clipping is aggravated in automotive audio systems. Less voltage headroom (i.e., safety margin) is available to the power amplifier since the automobile is limited to a 12-volt electrical supply. Although a DC/DC converter can be used to obtain a higher DC voltage, such converters are relatively expensive. Also, bass boost is needed in the automotive environment to overcome low frequency road and engine noise, making clipping more likely in the bass range of the audio signal.
It is known to employ voltage limiting or compression to the input of an amplifier to limit the occurrence of clipping (but some amount of clipping up to the limit is desirable; otherwise it may seem that the audio system does not play loud enough). In prior art voltage limiters, the amplifier gain is reduced when the power amplifier exhibits a specific percentage of total harmonic distortion (THD), typically about 10%. The 10% THD value represents a desirable amount of allowable distortion for typical full-band (i.e., wideband) audio material such as FM broadcasts, CD media, or cassette tape media.
Wideband audio material has significant spectral content at high (i.e., treble) frequencies. This treble content “masks” or makes less audible the distortion occurring at lower frequencies because of the peculiarities of human auditory perception. For narrower bandwidth signals (e.g., AM broadcasts or recording of piano solos) with less treble frequency content, however, this auditory masking does not occur so that the reproduced audio sounds significantly more distorted even though the actual amount of THD has not changed. Thus, prior art audio systems that sound fine while reproducing full-spectrum audio signals can sound very distorted while reproducing bandwidth-limited material such as an AM radio broadcast.
Prior art audio systems are known that separate the audio signal into separate bands for voltage limiting. After limiting, the separate bands must then be mixed back together in the output. The separating elements, additional signal processing paths, and the recombining elements add their own distortion to the signal and add significant expense to the audio system. Thus, it would be desirable to achieve distortion limiting of audio signals that is consistent with the psycho-acoustic effects of treble-frequency masking without requiring separate limiting in multiple frequency bands.